1. Field of the Invention
The present invention relates to an electrically conductive roll, such as a charging roll, a transfer roll and a cleaning roll, for use in electrophotographic copying machines, printers or the like, which conductive roll is required to exhibit low hardness or high flexibility, and electrical conductivity.
2. Discussion of the Related Art
An electrically conductive roll such as a charging roll, a transfer roll and a cleaning roll to be used in electrophotographic copying machines and printers is generally required to exhibit low hardness or high flexibility, and electrical conductivity. To meet this requirement, there has been conventionally employed a conductive roll as shown in the drawing, wherein a base layer consisting of an electrically conductive elastic body having low hardness and a predetermined thickness is formed on the outer circumferential surface of a suitable center shaft (metal core). On the outer surface of the base layer, there are formed by coating the following layers in the order of description: an electrode layer having a relatively small thickness; a dielectric layer (resistance adjusting layer); and a protective layer. In the case of a charging roll with a center shaft (10) having a diameter of about 6 mm, for example, a base layer (12) which has a thickness of about 3 mm is formed on the outer surface of the center shaft (10). Then, on the outer surface of the base layer (12), there are formed an electrode layer (14), a dielectric layer (16), and a protective layer (18) in the order of description which have a thickness of about 10 .mu.m, 160 .mu.m, and 10 .mu.m, respectively.
The base layer of the conventional conductive roll as described above is formed of an electrically conductive elastic material prepared by adding an electron-conductive material, such as a carbon black or metal powder, to an elastic material (major component) such as EPDM (ethylene propylene diene monomer), SBR (styrene-butadiene rubber), or NR (natural rubber), so that the prepared mixture is adjusted to have a volume resistivity of not more than 10.sup.3 .OMEGA.cm. To the thus prepared mixture, there is further added a relatively large amount of softener such as a process oil or a liquid polymer, so that the mixture has a JIS A type (Japanese Industrial Standards) hardness value (Hs) as low as 30.degree.. Thus, the material for the base layer prepared as described above provides the conductive roll with low hardness (high flexibility) and high electrical conductivity.
The electrode layer formed on the outer surface of the base layer consists of a material which is prepared by mixing carbon black with a synthetic resin such an nylon, so as to have a volume resistivity of around 10.sup.2 .OMEGA.cm. The electrode layer assures resistance uniformity of the conductive roll, and serves as a barrier for inhibiting the softener contained in the material of the base layer from migrating into the dielectric and protective layers, and consequently onto the outer surface of the roll. The dielectric layer formed on the outer surface of the electrode layer is formed of a material including epichlorohydrin rubber as a major constituent, for instance. The material is formulated to have a volume resistivity of around 10.sup.7 .OMEGA.cm, so that the electrode layer formed of the thus prepared material is capable of adjusting the resistance of the conductive roll and preventing leakage of an electric current therethrough. The protective layer provided on the dielectric layer is formed of a material prepared by mixing an electrically conductive powder of a solid solution of antimony oxide and stannic oxide, so as to have a volume resistivity of around 10.sup.7 .OMEGA.cm. The protective layer formed of the thus prepared material prevents the conductive roll from sticking to a photosensitive or photoconductive body such as a photoconductive drum used in a photocopying machine, for instance.
In the conventional conductive roll as described above, however, since the base layer is formed of the electrically conductive elastic composition which includes an electron-conductive material to give the base layer having a volume resistivity of not more than 10.sup.3 .OMEGA.cm, the conductive roll suffers from a problem that the level of the voltage at which an abnormal electric discharge is induced is undesirably low. Accordingly, the conventional conductive roll tends to suffer from the abnormal electric discharge upon application of a relatively high voltage for reproducing an image on a copy sheet. As a result, the image reproduced by using the conventional conductive roll has a low copy quality, that is, lines undesirably appear as a part of the reproduced image in the transverse direction of the copy sheet. In the conventional conductive roll, both of the base and electrode layers have relatively low volume resistivity values. Accordingly, the resistance of the conductive roll to the dielectric breakdown (and consequent current leakage) is highly dependent on the dielectric layer in the form of a relatively thin coating layer. As a result, the dielectric layer is likely to be subject to the dielectric breakdown at its local portions having a comparatively small thickness, leading to an insufficient operating reliability of the conductive roll. Furthermore, the hardness of the conductive roll is liable to increase since the electrically conductive elastic composition which gives the base layer of the conventional conductive roll contains the electron-conductive material such as a carbon black or metal powder dispersed in the matrix of the elastic material. For assuring a suitable or desired nip between the conductive roll and the photoconductive drum when the roll is in pressed contact with the drum, the material for the base layer must unfavorably contain an excessively large amount of the softener to reduce the hardness of the conductive roll.